I got my Lumia 620 today and I'm well impressed! Bear in mind I'm new to these things so I guess I'm a little bit easily pleased. Still, it's great fun just playing with Construct and seeing what I can do.
Some tests. I did a physics object test, just randomly creating rocks which fall down into a pit. On my PC, it can easily fill the entire screen without any slow-down, but on the phone it managed about 20 rocks before it started to struggle, and by about 40 rocks it would be 'unplayable'. Still, I think that's quite good since physics objects are fairly stressful on a processor, especially as these had about 8 points.
Sprites weren't so good though. I just had 64x64 pixel sprites being created and it started to slow down faster than I'd hoped - after around 70 sprites it dropped below 60fps and reached 30fps at about 150 sprites.
Still, great fun to play around with and I'm sure it can handle simpler games.